1. Field of the Invention
The present invention relates to the field of ht swap circuits.
2. Prior Art
In many systems, it is desired to be able to replace a printed circuit board without shutting off power to the system. This is typically referred to as xe2x80x9chot swapping.xe2x80x9d In computer systems, hot swapping will allow the replacement of a board or the addition of a new board in the system without loss of information in volatile memory in the system, without requiring rebooting the system, etc. In systems such as communication systems and the like, wherein a plurality of boards of similar function are plugged into a motherboard, boards may be hot swapped or additional boards added without shutting down the system. This allows maintenance and upgrading without interfering with communications or other functions in channels serviced by the remaining boards in the system. In hot swapping applications, however, unless inrush currents are adequately limited, the addition of a board to a system in operation can cause a momentary power glitch that may disturb other circuits in the system.
The present invention pertains to ways and means of controlling the maximum current supplied to a load from a power line, as may be useful in hot swapping applications. Overcurrent conditions can be encountered either by temporal inrush currents that happen on load insertion, or by a more permanent fault condition on the load, like a short circuit. Such situations are known as xe2x80x9chot springsxe2x80x9d. Classical hot swap circuits are basically controlled switches that connect the load to the power supply line. Hot swap circuits provide a way to soft start the power line connection in order to limit the inrush current absorbed by a capacitive load. Also they allow turning the switch off if the current through the switch exceeds some pre-determined value. While the latter function is usually well controlled, the limitation of the inrush current during the start-up phase largely depends on the load. Most of the hot swap circuits now in use are slew-rate controllers, and can only work with certain load values.
Hot swap current limit circuits and methods that provide soft start, current limiting and thermal limiting are disclosed. The circuits incorporate both a fast current limit loop to limit current spikes, and a slower current limit loop to more accurately limit the maximum or limit current to a predetermined value for longer periods, such as during start up or prolonged periods of excessive load conditions. Details of an exemplary embodiment circuit and method suitable for realization entirely in integrated circuit form are disclosed.